U.S. patent application number 11/962199 was filed with the patent office on 2008-07-03 for discharge lamp lighting circuit.
This patent application is currently assigned to KOITO MANUFACTURING CO., LTD.. Invention is credited to Tomoyuki ICHIKAWA, Takao MURAMATSU.
Application Number | 20080157692 11/962199 |
Document ID | / |
Family ID | 39477878 |
Filed Date | 2008-07-03 |
United States Patent
Application |
20080157692 |
Kind Code |
A1 |
ICHIKAWA; Tomoyuki ; et
al. |
July 3, 2008 |
DISCHARGE LAMP LIGHTING CIRCUIT
Abstract
A discharge lamp lighting circuit is provided. The discharge
lamp lighting circuit includes an inverter circuit which has two
output ends; a series resonant circuit which includes a capacitor,
an inductor and a transformer, coupled in series; a driving
portion; and a controlling portion which provides a control signal
for controlling said inverter circuit, said controlling portion
including a first signal producing portion which produces a first
signal indicative of a phase of a current flowing through said
series resonant circuit; and a second signal producing portion
which produces a second signal indicative of a phase of the AC
voltage output from said inverter circuit, said controlling portion
producing the control signal on the basis of a phase difference
between the first and second signals, wherein one component of said
series resonant circuit is coupled between one of said output ends,
and a detection point.
Inventors: |
ICHIKAWA; Tomoyuki;
(Shizuoka-shi, JP) ; MURAMATSU; Takao;
(Shizuoka-shi, JP) |
Correspondence
Address: |
SUGHRUE-265550
2100 PENNSYLVANIA AVE. NW
WASHINGTON
DC
20037-3213
US
|
Assignee: |
KOITO MANUFACTURING CO.,
LTD.
Tokyo
JP
|
Family ID: |
39477878 |
Appl. No.: |
11/962199 |
Filed: |
December 21, 2007 |
Current U.S.
Class: |
315/219 |
Current CPC
Class: |
Y02B 20/00 20130101;
Y02B 20/202 20130101; H05B 41/2887 20130101 |
Class at
Publication: |
315/219 |
International
Class: |
H05B 37/02 20060101
H05B037/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2006 |
JP |
2006-352776 |
Claims
1. A discharge lamp lighting circuit comprising: an inverter
circuit which has two output ends, and which outputs an AC voltage
between said two output ends; a series resonant circuit which
comprises a capacitor, at least one inductor and at least one
transformer, and which supplies AC power to said discharge lamp,
wherein said capacitor, said at least one inductor and said at
least one transformer are coupled in series between said two output
ends of said inverter circuit; a driving portion which drives said
inverter circuit; and a controlling portion which provides said
driving portion with a control signal for controlling said inverter
circuit, said controlling portion comprising: a first signal
producing portion which produces a first signal indicative of a
phase of a current flowing through said series resonant circuit
based on a voltage at a detection point in said series resonant
circuit; and a second signal producing portion which produces a
second signal indicative of a phase of the AC voltage output from
said inverter circuit, said controlling portion producing the
control signal on the basis of a phase difference between the first
and second signals, wherein one component of said inductor, said
transformer, and said capacitor which comprise the series resonant
circuit, is coupled between one of said two output ends, and said
detection point.
2. A discharge lamp lighting circuit according to claim 1, wherein
said one component is said capacitor.
3. A discharge lamp lighting circuit according to claim 1, wherein
said one component is said at least one inductor.
4. A discharge lamp lighting circuit according to claim 1, wherein
said first signal producing portion comprises: a differentiating
circuit which differentiates a voltage at said detection point; and
a converting circuit which converts an output of said
differentiating circuit to a digital signal.
5. A discharge lamp lighting circuit according to claim 1, wherein
said first signal producing portion comprises: an integrating
circuit which integrates a voltage at said detection point; and a
converting circuit which converts an output of said integrating
circuit to a digital signal.
6. A discharge lamp lighting circuit according to claim 1, wherein
said first signal producing portion comprises: a first circuit
which performs one of integration and differentiation on a voltage
at said detection point; a second circuit which performs one of
integration and differentiation not performed by the first circuit
on an output of said first circuit; and a converting circuit which
converts an output of said second circuit to a digital signal.
7. A discharge lamp lighting circuit comprising: an inverter
circuit which has two output ends; a series resonant circuit which
comprises at least three components, said at least three components
coupled together in series between said two output ends of said
inverter circuit; a driving portion which drives said inverter
circuit; and a controlling portion which provides said driving
portion with a control signal for controlling said inverter
circuit, said control signal generated based on a difference
between a phase of a current based on a voltage at a detection
point in said series resonant circuit and a phase of an AC voltage
output from said inverter circuit.
8. A discharge lamp lighting circuit according to claim 7, wherein
said detection point is located at an end of one component of said
at least three components which is opposite to an end of the one of
said at least three components which is coupled to said inverter
circuit.
9. A discharge lamp lighting circuit according to claim 8, wherein
said one component is said capacitor.
10. A discharge lamp lighting circuit according to claim 8, wherein
said one component is said at least one inductor.
11. A discharge lamp lighting circuit according to claim 7, wherein
said controlling portion comprises: a differentiating circuit which
differentiates a voltage at said detection point; and a converting
circuit which converts an output of said differentiating circuit to
a digital signal.
12. A discharge lamp lighting circuit according to claim 7, wherein
said controlling portion comprises: an integrating circuit which
integrates a voltage at said detection point; and a converting
circuit which converts an output of said integrating circuit to a
digital signal.
13. A discharge lamp lighting circuit according to claim 7, wherein
said controlling portion comprises: a first circuit which performs
one of integration and differentiation on a voltage at said
detection point; a second circuit which performs one of integration
and differentiation not performed by the first circuit on an output
of said first circuit; and a converting circuit which converts an
output of said second circuit to a digital signal.
14. A lighting device for a vehicle, comprising: a discharge lamp
lighting circuit comprising: an inverter circuit which has two
output ends, and which outputs an AC voltage between said two
output ends; a series resonant circuit which comprises a capacitor,
at least one inductor and at least one transformer, and which
supplies AC power to said discharge lamp, wherein said capacitor,
said at least one inductor and said at least one transformer are
coupled in series between said two output ends of said inverter
circuit; a driving portion which drives said inverter circuit; and
a controlling portion which provides said driving portion with a
control signal for controlling said inverter circuit, said
controlling portion comprising: a first signal producing portion
which produces a first signal indicative of a phase of a current
flowing through said series resonant circuit based on a voltage at
a detection point in said series resonant circuit; and a second
signal producing portion which produces a second signal indicative
of a phase of the AC voltage output from said inverter circuit,
said controlling portion producing the control signal on the basis
of a phase difference between the first and second signals, wherein
said capacitor is coupled between one of said two output ends, and
said detection point, and wherein said first signal producing
portion comprises: a first circuit which performs one of
integration and differentiation on a voltage at said detection
point; a second circuit which performs one of integration and
differentiation not performed by the first circuit on an output of
said first circuit; and a converting circuit which converts an
output of said second circuit to a digital signal.
Description
[0001] This application claims priority from Japanese Patent
Application No. 2006-352776, filed Dec. 27, 2006, in the Japanese
Patent Office. Japanese Patent Application No. 2006-352776 is
hereby incorporated by reference in its entirety.
TECHNICAL FIELD
[0002] Apparatuses consistent with the present invention relate to
a discharge lamp lighting circuit.
RELATED ART
[0003] In order to light a discharge lamp such as a metal halide
lamp used as a headlamp for a vehicle, a lighting circuit (i.e., a
ballast) for stably supplying a power to the lamp is used. For
example, Japanese Patent Unexamined Publication No. 2005-63823
shows a related art discharge lamp lighting circuit which comprises
a DC-AC converting circuit including a series resonant circuit. The
DC-AC converting circuit supplies an AC power to a discharge lamp.
The level of the supplied power is controlled by changing a driving
frequency of a bridge driver which drives the series resonant
circuit.
[0004] When the driving frequency of the series resonant circuit is
to be changed, one option is to control the driving frequency on
the basis of a phase difference between a voltage and current of
the series resonant circuit. In this case, when a maximum power of
the series resonant circuit is to be supplied while making the
driving frequency of the bridge driver coincident with the resonant
frequency of the series resonant circuit, for example, the driving
frequency may be controlled so that the phase difference between
the voltage and current of the series resonant circuit approaches
zero.
[0005] In such a case, it is advantageous to individually detect
the phases of the voltage and current of the series resonant
circuit. One related art option for detecting the phase of a
current uses a transformer inserted into the series resonant
circuit. The current is then detected from the secondary side of
the transformer. However, in a case where the resonant frequency of
the series resonant circuit becomes high, for example, 2 MHz, the
iron loss of the current detection transformer becomes large.
Furthermore, the number of components of the circuit is increased,
so that a size and a production cost are increased.
[0006] Another option for detecting the phase of a current of a
series resonant circuit is to use a resistor. A resistor for
detecting a current is inserted into the series resonant circuit,
and the phase of a current is detected from the waveform of a
voltage across the resistor. In a discharge lamp lighting circuit,
the level of a current flowing through a series resonant circuit
varies greatly in a range of several hundreds of mA to 100 A. Thus,
when the resistance of the resistor for detecting the current is
increased in order to ensure a high detection accuracy in the case
of a lower current, the power loss in the case of a higher current
becomes excessively large. By contrast, when the resistance of the
resistor for detecting a current is decreased in order to reduce
the power loss in the case of a higher current, a high detection
accuracy in the case of a lower current cannot be ensured.
[0007] In order to address some of these problems, yet another
option has been suggested in the related art. In the secondary side
of a transformer for transmitting an AC power of a series resonant
circuit to a discharge lamp (i.e., the side to which the discharge
lamp is coupled), a resistor for detecting a current is coupled in
series to the discharge lamp, a current (i.e., a lamp current)
flowing through the discharge lamp is detected from a voltage
across the resistor, and a phase of this current is determined used
as the current of the series resonant circuit. However, this option
also has problems. First, detection is disabled during a period
when the discharge lamp is not lighted. Second, since the lamp
current is very small immediately after the lighting of the
discharge lamp, detection is also disabled during a period when the
discharge lamp is transferred from a glow discharge to an arc
discharge, and hence the driving frequency cannot be controlled
during this period. Lastly, as a practical matter, the phase of the
current of the series resonant circuit is not coincident with that
of the lamp current, and hence the difference of the two phases may
adversely affect the control of the driving frequency.
BRIEF SUMMARY OF THE PRESENT INVENTION
[0008] Exemplary embodiments of the present invention provide a
discharge lamp lighting circuit in which a transformer or resistor
for detecting a current is not used in a series resonant circuit,
and the phase of the current of the series resonant circuit can be
accurately detected even in a state where a discharge lamp is not
lighted, or an arc discharge has not yet occurred.
[0009] According to an aspect of the present invention, a discharge
lamp lighting circuit is provided which supplies an AC power for
lighting a discharge lamp, to the discharge lamp, wherein the
discharge lamp lighting circuit comprises an inverter circuit which
has two output ends, and which outputs an AC voltage between the
two output ends; a series resonant circuit which includes a
capacitor and at least one of an inductor and a transformer, and
which supplies the AC power to the discharge lamp, the capacitor
and the at least one of the inductor and the transformer being
coupled in series between the two output ends of the inverter
circuit; a driving portion which drives the inverter circuit; and a
controlling portion which provides the driving portion with a
control signal for controlling a driving frequency of the inverter
circuit. The controlling portion comprises a first signal producing
portion which produces a first signal indicative of a phase of a
current flowing through the series resonant circuit; and a second
signal producing portion which produces a second signal indicative
of a phase of the AC voltage output from the inverter circuit, the
controlling portion producing the control signal on the basis of a
phase difference between the first and second signals. The first
signal producing portion produces the first signal on the basis of
a voltage at a detection point in the series resonant circuit, and
one of the inductor, the transformer, and the capacitor is coupled
between one of the two output ends, and the detection point.
[0010] In the discharge lamp lighting circuit, the controlling
portion which controls the driving frequency of the inverter
circuit may include the first signal producing portion for
detecting the phase of the current flowing through the series
resonant circuit; and the second signal producing portion for
detecting the phase of the AC voltage output from the inverter
circuit. The controlling portion may control the driving frequency
on the basis of the phase difference between the current flowing
through the series resonant circuit and the AC voltage. According
to this exemplary configuration, for example, the driving frequency
is controlled so that a phase difference between the voltage and
current of the series resonant circuit approaches zero, and the
driving frequency of the inverter circuit is made coincident with
the resonant frequency of the series resonant circuit, so that the
maximum power of the series resonant circuit can be supplied to the
discharge lamp.
[0011] In the discharge lamp lighting circuit, one of the inductor,
the transformer, and the capacitor is coupled between one of the
two output ends of the inverter circuit, and the detection point of
the first signal producing portion. When the voltage of the plus
output end of the inverter circuit is indicated by Va, and that of
the position between which and the output end one of the inductor,
the transformer, and the capacitor is coupled is indicated by Vb,
the relationship between the voltages Va and Vb is expressed by
following Expression (1). In Expression (1), Z denotes an impedance
of the inductor, the transformer, or the capacitor, and I denotes a
current flowing through the series resonant circuit.
[0012] [Exp. 1]
Vb=Va-ZI (1)
When the voltage of the minus output end of the inverter circuit is
indicated by Vc, and that of the position between which and the
output end one of the inductor, the transformer, and the capacitor
is coupled is indicated by Vd, the relationship between the
voltages Vc and Vd is expressed by following Expression (2).
[0013] [Exp. 2]
Vd=Vc+ZI (2)
In Expression (1), the potential Va denotes a plus output of the
inverter circuit, and either of the power source voltage and the
ground potential. In Expression (2), the potential Vc denotes a
minus output of the inverter circuit, and the ground potential.
Therefore, the value of the current I is obtained from the value of
the voltage Vb according to Expression (1), or from the value of
the voltage Vd according to Expression (2). In other words, the
phase of the current can be known by detecting the voltage Vb (or
Vd) at the detection point in the series resonant circuit.
[0014] In the discharge lamp lighting circuit, namely, the phase of
the current may be obtained by referring to the potential at the
detection point in the series resonant circuit. Therefore, a
transformer or resistor for detecting a current is not used in the
series resonant circuit, and the phase of the current of the series
resonant circuit can be accurately detected even in a state where
the discharge lamp is not lighted, or an arc discharge has not yet
occurred.
[0015] Furthermore, the discharge lamp lighting circuit may be
characterized in that the capacitor is coupled between the
detection point and the one output end. Usually, an inverter
circuit is configured by a transistor, and an element which is of
the surface mount type and which has a small size is often used as
the transistor. Similarly, a capacitor which is of the surface
mount type, and which is relatively smaller than an inductor and a
transformer can be used. When a capacitor is disposed in place of
an inductor or a transformer between the detection point of the
first signal producing portion and the one output end of the
inverter circuit, the current path of the series resonant circuit
can be shortened, and the high-frequency characteristic of the
series resonant circuit can be stabilized. Furthermore, a
transistor and capacitor which are small in size can be disposed
close to each other, so that the space on a circuit board can be
efficiently used.
[0016] Furthermore, the discharge lamp lighting circuit may be
characterized in that the first signal producing portion comprises
a differentiating circuit which differentiates the voltage at the
detection point; and a converting circuit which converts an output
signal of the differentiating circuit to a digital signal.
Alternatively, the discharge lamp lighting circuit may be
characterized in that the first signal producing portion comprises
an integrating circuit which integrates the voltage at the
detection point; and a converting circuit which converts an output
signal of the integrating circuit to a digital signal.
[0017] According to an aspect of the present invention, in the
discharge lamp lighting circuit, one of the inductor, the
transformer, and the capacitor is coupled between the output end of
the inverter circuit, and the detection point of the first signal
producing portion. In a state where an arc discharge has not yet
occurred, for example, the impedance of the discharge lamp is high,
and hence the current I of the series resonant circuit is large. In
such a state, the phase of the voltage Vb (Vd) at the detection
point leads (or lags) by about 90.degree. the current I. Also in
the case where the driving frequency is controlled so that the
phase difference between the voltage and current of the series
resonant circuit approaches zero, and the driving frequency of the
inverter circuit is made coincident with the resonant frequency of
the series resonant circuit, the phase of the voltage Vb (Vd) leads
(or lags) by about 90.degree. the current I. In these cases, when
the voltage at the detection point is differentiated (or
integrated), the phase difference between the differentiated (or
integrated) voltage and the current I is about 0.degree. or about
180.degree.. Therefore, the phase difference between the voltage Va
after being converted to a digital signal and the current I can be
easily processed.
[0018] Furthermore, the discharge lamp lighting circuit may be
characterized in that the first signal producing portion comprises
a first circuit which performs one of integration and
differentiation on the voltage at the detection point; a second
circuit which performs another one of integration and
differentiation on an output signal of the first circuit; and a
converting circuit which converts an output signal of the second
circuit to a digital signal.
[0019] In the case where the voltage at the detection point is
differentiated by the differentiating circuit, the differentiating
circuit cuts the DC component of an input signal, and hence
detection of a zero crossing of the voltage at the detection point
is highly accurate. When the voltage at the detection point
contains high-frequency noise, however, the differentiating circuit
tends to allow the noise components to pass therethrough because
the gain is higher as the frequency is higher, and hence erroneous
detection may be caused. By contrast, in the case where the voltage
at the detection point is integrated by the integrating circuit,
when the input signal contains high-frequency noise, the
integrating circuit cuts the noise components, and the signal to
noise (S/N) ratio can be improved. In order to realize a phase lag,
however, the gain is excessively lowered, and there is a
possibility that a signal cannot be detected. When the integrating
and differentiating circuits are combined with each other and the
circuits are set to respective adequate cutoff frequencies, the S/N
ratio can be improved while the accuracy of detection of a zero
crossing of the voltage at the detection point is enhanced.
[0020] Other aspects will be apparent from the following detailed
description, the accompanying drawings and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a block diagram showing a discharge lamp lighting
circuit according to an exemplary embodiment of the present
invention;
[0022] FIG. 2 is a block diagram showing an example of an internal
configuration of a controlling portion of the discharge lamp
lighting circuit of FIG. 1;
[0023] FIGS. 3(a) to 3(f) are graphs exemplarily showing temporal
variations of waveforms in a case where a series resonant circuit
operates in an inductive region, and indicating phase relationships
of the waveforms;
[0024] FIGS. 4(a) to 4(f) are graphs exemplarily showing temporal
variations of waveforms in a case where a series resonant circuit
operates in a capacitive region, and indicating phase relationships
of the waveforms;
[0025] FIG. 5 is a circuit diagram showing an equivalent circuit of
resonance portion of a series resonant circuit;
[0026] FIGS. 6(a) to 6(d) are graphs showing transitions of a
driving frequency of a bridge driver, a voltage of a starting
capacitor, and a lamp voltage, respectively, of the discharge lamp
lighting circuit of FIG. 1;
[0027] FIGS. 7(a) and 7(c) are graphs showing transitions of a
control signal and a supplied power, respectively, of the discharge
lamp lighting circuit of FIG. 1;
[0028] FIG. 8 is a graph showing transition of relationships
between the driving frequency of the series resonant circuit and
the supplied power of the discharge lamp lighting circuit of FIG.
1;
[0029] FIG. 9(a) is a block diagram showing an example of internal
configurations of a frequency following controlling portion, and
first and second signal producing portions according to an
exemplary embodiment of the present invention, and FIG. 9(b) is a
view showing an example of a circuit configuration of a
differentiating circuit according to an exemplary embodiment of the
present invention;
[0030] FIG. 10 is a view diagrammatically showing a configuration
of a discharge lamp lighting circuit according to an exemplary
embodiment of the present invention;
[0031] FIG. 11(a) is a block diagram of internal configurations of
the frequency following controlling portion, and the first and
second signal producing portions according to another exemplary
embodiment of the present invention, and FIG. 11(b) is a view
showing an example of the circuit configuration of an integrating
circuit according to an exemplary embodiment of the present
invention;
[0032] FIG. 12(a) is a block diagram of internal configurations of
the frequency following controlling portion, and the first and
second signal producing portions according to another exemplary
embodiment of the present invention, and FIG. 12(b) is a view
showing an example of the circuit configuration of an integrating
circuit and a differentiating circuit according to another
exemplary embodiment of the present invention;
[0033] FIG. 13 is a view showing a discharge lamp lighting circuit
according to another exemplary embodiment of the present invention;
and
[0034] FIG. 14 is a view showing a discharge lamp lighting circuit
according to yet another exemplary embodiment of the present
invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT OF THE PRESENT
INVENTION
[0035] Hereinafter, exemplary embodiments of the discharge lamp
lighting circuit of the invention will be described in detail with
reference to the accompanying drawings. In the description of the
drawings, identical parts are denoted by the same reference
numerals, and their duplicated description will be omitted.
[0036] FIG. 1 is a block diagram showing a configuration of a
discharge lamp lighting circuit according to an exemplary
embodiment of the present invention. The discharge lamp lighting
circuit 1 shown in FIG. 1 supplies an AC power for lighting a
discharge lamp L, to the discharge lamp L, or converts a DC voltage
VB from a DC power source B to an AC voltage, and supplies the AC
voltage to the discharge lamp L. The discharge lamp lighting
circuit 1 is used mainly in a lighting device for a vehicle, such
as a headlamp. However, the discharge lamp lighting circuit 1 may
also be used with other lighting devices in other applications
using similar lighting devices. As the discharge lamp L, for
example, a mercury-free metal halide lamp may be used. However,
discharge lamps having another structures may also be used with the
discharge lamp lighting circuit according to exemplary embodiments
of the invention.
[0037] The discharge lamp lighting circuit 1 comprises a power
supplying portion 2, a controlling portion 10, and a voltage to
frequency (V-F) converting portion 24. The power supplying portion
2 receives a power supply from the DC power source B, and supplies
the AC power to the discharge lamp L. The controlling portion 10
controls the level of the power to be supplied to the discharge
lamp L. The V-F converting portion 24 performs voltage-frequency
conversion (V-F conversion) on a control signal Sc.sub.1 which is
an analog signal supplied from the controlling portion 10, to
produce a control signal Sc.sub.2.
[0038] The power supplying portion 2 supplies a power the level of
which is based on the control signal Sc.sub.2 supplied from the
controlling portion 10, to the discharge lamp L. The power
supplying portion 2 is coupled to the DC power source B (such as a
battery) via a switch 20 for a lighting operation, to receive the
DC voltage VB from the DC power source B, and performs AC
converting and voltage boosting operations. In this exemplary
embodiment, the power supplying portion 2 comprises a half-bridge
inverter circuit (hereinafter, referred to simply as an inverter
circuit) 3 which converts the DC voltage VB to an AC voltage of a
rectangular wave; a series resonant circuit 4 which is disposed in
a subsequent stage of the inverter circuit 3; a starting portion 5
which, at the start of lighting, applies a high-voltage pulse to
the discharge lamp L to promote lighting; and a bridge driver 6
which is a driving portion for driving the inverter circuit 3.
[0039] The inverter circuit 3 has two output ends 3a, 3b, and
outputs the AC voltage of a rectangular wave between the output
ends 3a, 3b. The inverter circuit 3 is configured by coupling in
series two transistors 31, 32 which are switching elements.
Specifically, one current terminal (drain terminal) of the
transistor 31 is coupled to a plus terminal of the DC power source
B through the switch 20, and the other current terminal (source
terminal) of the transistor 31 is coupled to one current terminal
(drain terminal) of the transistor 32. The control terminal (gate
terminal) of the transistor 31 is coupled to the bridge driver 6.
The other current terminal (source terminal) of the transistor 32
is coupled to a ground potential line GND (i.e., a minus terminal
of the DC power source B), and the control terminal (gate terminal)
of the transistor 32 is coupled to the bridge driver 6. The output
end 3a of the inverter circuit 3 is taken from the source terminal
of the transistor 31 (also the drain terminal of the transistor
32), and the output end 3b is taken from the source terminal of the
transistor 32. The bridge driver 6 supplies drive signals
Sdrv.sub.1, Sdrv.sub.2 which are opposite in phase to each other,
to the gate terminals of the transistors 31, 32, respectively,
thereby causing the transistors 31, 32 to be alternatingly
conductive. As the transistors 31, 32, N-channel metal oxide field
effect transistors (MOSFETs) are used, as shown for example in FIG.
1. However, other FETs or bipolar transistors may also be used.
[0040] The series resonant circuit 4 comprises a transformer 7, a
capacitor 8, and an inductor 9. The transformer 7 is disposed so as
to apply a high-voltage pulse to the discharge lamp L, transmit the
power, and boost the power. The transformer 7, the capacitor 8, and
the inductor 9 constitute a series resonant circuit. Namely, the
capacitor 8, the inductor 9, and a primary winding 7a of the
transformer 7 are coupled in series. An end of the series circuit
on the side of the capacitor 8 is coupled to the one output end 3a
of the inverter circuit 3, and an other end on the side of the
primary winding 7a is coupled to the other output end 3b of the
inverter circuit 3. According to this exemplary configuration, the
resonant frequency is determined by a combined reactance configured
by the leakage inductance of the primary winding 7a of the
transformer 7, and the inductance of the inductor 9, and the
capacitance of the capacitor 8. Alternatively, the series resonant
circuit may be configured only by the primary winding 7a and the
capacitor 8, and the inductor 9 may be omitted. Alternatively, the
inductance of the primary winding 7a may be set to be much smaller
than that of the inductor 9, and the resonant frequency may be
determined substantially by the inductor 9 and the capacitance of
the capacitor 8.
[0041] In the inverter circuit 3 and the series resonant circuit 4,
using the series resonance phenomenon due to the capacitor 8 and
the inductive elements (the inductance component and the inductor),
the transistors 31, 32 are alternatingly turned on and off while
the driving frequency of the transistors 31, 32 is set to a value
which is equal to or higher than the series resonant frequency,
thereby causing an AC power to be produced in the primary winding
7a of the transformer 7. The AC power is transmitted to a secondary
winding 7b of the transformer 7 while being boosted, and then
supplied to the discharge lamp L coupled to the secondary winding
7b. The bridge driver 6 which drives the transistors 31, 32
complementarily drives the transistors 31, 32 so that both the
transistors 31, 32 are not simultaneously in the conductive
state.
[0042] In the series resonant circuit 4, the series resonant
frequency fa before lighting, and the series resonant frequency fb
after lighting are expressed by the following Expressions (3) and
(4), respectively. In the Expressions, C denotes the capacitance of
the capacitor 8, Lr denotes the inductance of the inductor 9, Lp1
denotes the inductance of the primary winding 7a before lighting,
and Lp2 denotes the inductance of the primary winding 7a after
lighting.
fa = 1 2 .pi. C .times. ( Lr + Lp 1 ) [ Exp . 3 ] fb = 1 2 .pi. C
.times. ( Lr + Lp 2 ) [ Exp . 4 ] ##EQU00001##
It is advantageous to select Lp1>Lp2. Therefore, the series
resonant frequency fa before lighting is lower than the series
resonant frequency fb after lighting.
[0043] The impedance of the series resonant circuit 4 is changed in
accordance with the driving frequency of the transistors 31, 32 by
the bridge driver 6. Therefore, the level of the AC power to be
supplied to the discharge lamp L can be controlled by changing the
driving frequency. Namely, the level of the power supplied to the
discharge lamp L has a maximum value when the driving frequency is
equal to the series resonant frequency, and is further decreased as
the driving frequency moves away (either above or below the series
resonant frequency. When the driving frequency is lower than the
series resonant frequency, however, a switching loss is large and
the power efficiency is reduced. Therefore, the magnitude of the
driving frequency of the bridge driver 6 is advantageously
controlled within a region where the driving frequency is higher
than the series resonant frequency. The region where the frequency
is lower than the series resonant frequency is referred to as a
capacitive region, and that where the frequency is higher than the
series resonant frequency is referred to as an inductive region. In
this exemplary embodiment, the driving frequency of the bridge
driver 6 is controlled in accordance with a pulse frequency of the
control signal Sc.sub.2 (a signal including a frequency-modulated
pulse train) supplied from the V-F converting portion 24 coupled to
the bridge driver 6.
[0044] The starting portion 5 is a circuit for applying the
high-voltage pulse for starting to the discharge lamp L. When the
starting portion 5 applies a trigger voltage and current to the
transformer 7, the high-voltage pulse is superimposed on the AC
voltage produced in the secondary winding 7b of the transformer 7.
The starting portion 5 comprises a starting capacitor (capacitive
element) 51 which stores a power for producing the high-voltage
pulse; and a self-breakdown switching element 52 such as a spark
gap or a gas arrester. One end of the starting capacitor 51 is
coupled to one end of an auxiliary winding 7c of the transformer 7
via a rectifying element (i.e., a diode) 53 and a resistor element
54, to provide the starting portion 5 with the input voltage. Both
the other ends of the auxiliary winding 7c and the starting
capacitor 51 are coupled to the output end 3b of the inverter
circuit 3 (i.e., the ground potential line GND). Alternatively, the
input voltage of the starting portion 5 may be obtained, for
example, from the secondary winding 7b of the transformer 7, or
from an auxiliary winding which cooperates with the inductor 9 to
configure a transformer.
[0045] One end of the self-breakdown switching element 52 is
coupled to one end of the starting capacitor 51, and the other end
of the self-breakdown switching element 52 is coupled to the middle
of the primary winding 7a. In the starting portion 5, when the
across voltage of the starting capacitor 51 reaches the discharge
starting voltage of the self-breakdown switching element 52, the
self-breakdown switching element 52 is momentarily set to the
conductive state, thereby outputting the trigger voltage and
current.
[0046] The controlling portion 10 controls the driving frequency of
the bridge driver 6 (i.e., the level of the power to be supplied to
the discharge lamp L). The controlling portion 10 comprises input
ends 10a to 10d, and an output end 10e. In order to receive a
signal (hereinafter, referred to as a lamp voltage corresponding
signal) VS1 indicative of the amplitude of a lamp voltage VL of the
discharge lamp L, the input end 10a is coupled to an intermediate
tap of the secondary winding 7b via a peak-hold circuit 21. The
lamp voltage corresponding signal VS1 is set to be, for example,
0.35 times the peak value of the lamp voltage VL. The input end 10b
is coupled to one end of a resistor element 25 which is disposed
for detecting the lamp current IL of the discharge lamp L, via a
peak-hold circuit 22 and a buffer 23. The one end of the resistor
element 25 is further coupled to one electrode of the discharge
lamp L via an output terminal of the discharge lamp lighting
circuit 1, and the other end of the resistor element 25 is coupled
to the output end 3b (ground potential line GND) of the inverter
circuit 3. The buffer 23 outputs a signal (hereinafter, referred to
as a lamp current corresponding signal) IS1 indicative of the
amplitude of the lamp current IL.
[0047] The input end 10c is coupled to a detection point 4a in the
series resonant circuit 4. The potential at the detection point 4a
is supplied to the input end 10c as a signal IS2 for detecting the
phase of the current of the series resonant circuit 4. In this
exemplary embodiment, the detection point 4a is set between the
capacitor 8 and the inductor 9. Among the elements comprising the
series resonant circuit 4 (i.e., the capacitor 8, the inductor 9,
and the primary winding 7a of the transformer 7), only the
capacitor 8 is coupled between the output end 3a of the inverter
circuit 3 and the detection point 4a in this exemplary
embodiment.
[0048] The input end 10d is coupled to the output end 3a of the
inverter circuit 3. In order to detect the phase of the AC voltage
output from the inverter circuit 3, an output voltage Vout of the
inverter circuit 3 is supplied to the input end 10d as a signal
VS2. As indicated by the broken line in the figure, the input end
10d may be coupled, for example, to the connection between the
bridge driver 6 and the gate terminal of the transistor 32 (or the
transistor 31). In this case, the drive signal Sdrv.sub.2 (or the
drive signal Sdrv.sub.1) is supplied to the input end 10d. The
input end 10d may be coupled to any place as far as the phase of
the AC voltage output from the inverter circuit 3 can be detected
in the place.
[0049] The V-F converting portion 24 receives the control signal
Sc.sub.1 which is an analog signal, through the output end 10e of
the controlling portion 10, and applies the V-F conversion on the
control signal Sc.sub.1 to produce the control signal Sc.sub.2. In
this exemplary embodiment, the V-F converting portion 24 is
configured so that the pulse frequency of the control signal
Sc.sub.2 is higher as the input voltage (i.e., the voltage of the
control signal Sc.sub.1) is lower.
[0050] Next, an internal configuration of the controlling portion
10 in the embodiment will be described. FIG. 2 is a block diagram
showing an example of the internal configuration of the controlling
portion 10 according to an exemplary embodiment of the present
invention. As shown in FIG. 2, the controlling portion 10 comprises
a frequency following controlling portion 11 which produces a
control signal S1; a power controlling portion 12 which produces a
control signal S2; a selecting portion 13 which selectively
supplies one of the control signals S1, S2 to the output end 10e; a
first signal producing portion 14 which waveform-shapes the signal
IS2 and then supplies it to the frequency following controlling
portion 11; and a second signal producing portion 15 which
waveform-shapes the signal VS2 and then supplies it to the
frequency following controlling portion 11.
[0051] The frequency following controlling portion 11 produces the
control signal S1 which causes the phase difference between the
voltage and current of the series resonant circuit 4 to approach
zero. The frequency following controlling portion 11 comprises
input ends 11a, 11b and an output end 11c. The signal IS2 is
supplied to the input end 11a via the input end 10c of the
controlling portion 10. The signal VS2 is supplied to the input end
11b via the input end 10d of the controlling portion 10. The
frequency following controlling portion 11 produces the control
signal S1 so that the difference between the phase of the current
of the series resonant circuit detected on the basis of the signal
IS2, and that of the output voltage Vout of the inverter circuit 3
detected on the basis of the signal VS2 approaches zero, and
supplies the control signal S1 to the selecting portion 13.
[0052] Before lighting of the discharge lamp L, the power
controlling portion 12 produces a control signal S2 so that the
level of the open circuit voltage (OCV) to be supplied to the
discharge lamp L becomes close to a threshold value. The threshold
value may be predetermined. After lighting of discharge lamp L, the
power controlling portion 12 produces the control signal S2 so that
the level of the power to be supplied to the discharge lamp L
becomes close to a steady-state value in accordance with a time
function. The time function may be predetermined.
[0053] For example, the power controlling portion 12 produces the
control signal S2 so that, after the lighting of the discharge lamp
L, the level of the supplied power first becomes an initial value
(for example, 75 W) in accordance with the time function, and,
after a certain time, the level of the supplied power gradually
approaches from an initial value to the steady-state value (for
example, 35 W).
[0054] In this exemplary embodiment, as shown in FIG. 2, the power
controlling portion 12 has a power calculating portion 121 and an
error amplifier 122. The power calculating portion 121 comprises an
input end 121a which receives the lamp voltage corresponding signal
VS1 via the input end 10a of the controlling portion 10; and an
input end 121b which receives the lamp current corresponding signal
IS1 via the input end 10b of the controlling portion 10. Before
lighting of the discharge lamp L, the power calculating portion 121
produces an output voltage V1 so that the lamp voltage
corresponding signal VS1 indicative of the level of the OCV becomes
close to the threshold value, and, after the lighting of the
discharge lamp L, produces the output voltage V1 so that the level
of the supplied power becomes close to the steady-state value in
accordance with the time function, on the basis of the lamp voltage
corresponding signal VS1 and the lamp current corresponding signal
IS1. The output voltage V1 is supplied from an output end 121c of
the power calculating portion 121 to the inverting input terminal
of the error amplifier 122, via a resistor 123. The non-inverting
input terminal of the error amplifier 122 is coupled to a voltage
source 124 which produces a reference voltage V2. The reference
voltage may be predetermined. The output voltage from the error
amplifier 122 is provided to the selecting portion 13 as the
control signal S2.
[0055] The selecting portion 13 is configured, for example, by a
switch 131. Before the high-voltage pulse is applied to the
discharge lamp L by the starting portion 5, the switch 131 couples
an output end 12a of the power controlling portion 12 to the output
end 10e of the controlling portion 10. During a time period of
several milliseconds after application of the high-voltage pulse to
the discharge lamp L, the switch 131 couples the output end 11c of
the frequency following controlling portion 11 to the output end
10e of the controlling portion 10. After elapse of several
milliseconds from application of the high-voltage pulse, the switch
131 again couples the output end 12a of the power controlling
portion 12 to the output end 10e of the controlling portion 10.
Before application of the high-voltage pulse to the discharge lamp
L, therefore, the control signal S2 is output from the controlling
portion 10, and, during several milliseconds immediately after
application of the high-voltage pulse to the discharge lamp L,
therefore, the control signal S1 is output, and thereafter the
control signal S2 is again output. The controlling portion 10
supplies the thus selected control signal S1 or S2 to the V-F
converting portion 24 (see FIG. 1) as the control signal
Sc.sub.1.
[0056] The first signal producing portion 14 waveform-shapes the
signal IS2 to digitize the signal, thereby producing a signal S3.
The signal S3 indicates the phase of the current flowing through
the series resonant circuit 4. The first signal producing portion
14 comprises an input end 14a and an output end 14b. The signal IS2
is supplied to the input end 14a via the input end 10c of the
controlling portion 10. On the basis of the signal IS2 (i.e., the
voltage waveform at the detection point 4a in FIG. 1), the first
signal producing portion 14 produces the signal S3, and supplies
the signal S3 to the frequency following controlling portion 11
through the output end 14b.
[0057] The second signal producing portion 15 waveform-shapes the
signal VS2 to digitize the signal, thereby producing a signal S4.
The signal S4 indicates the phase of the voltage (AC voltage) Vout
output from the inverter circuit 3. The second signal producing
portion 15 comprises an input end 15a and an output end 15b. The
signal VS2 is supplied to the input end 15a via the input end 10d
of the controlling portion 10. On the basis of the signal VS2
(i.e., the AC voltage Vout), the second signal producing portion 15
produces the signal S4, and supplies the signal S4 to the frequency
following controlling portion 11 through the output end 15b.
[0058] The functions of the frequency following controlling portion
11, the first signal producing portion 14, and the second signal
producing portion 15 will be described in further detail below.
FIGS. 3(a) to 3(f) are graphs exemplarily showing temporal
variations of waveforms in a case where the series resonant circuit
4 operates in the inductive region, and indicating phase
relationships of the waveforms. FIG. 3(a) shows the on and off
states of the transistors 31, 32; FIG. 3(b) shows the waveform of
the signal VS2 (AC voltage Vout); FIG. 3(c) shows the waveform of
the signal S4; FIG. 3(d) shows the current waveform of the series
resonant circuit 4; FIG. 3(e) shows the waveform of the signal IS2
(i.e., the voltage waveform at the detection point 4a); and FIG.
3(f) shows the waveform of the signal S3. As shown in FIGS. 3(b)
and 3(d), in the inductive region, the current of the series
resonant circuit lags in phase the voltage.
[0059] FIG. 4 is a graph exemplarily showing temporal variations of
waveforms in a case where the series resonant circuit 4 operates in
the capacitive region, and indicating phase relationships of the
waveforms. FIG. 4(a) shows the on and off states of the transistors
31, 32; FIG. 4(b) shows the waveform of the signal VS2; FIG. 4(c)
shows the waveform of the signal S4; FIG. 4(d) shows the current
waveform of the series resonant circuit 4; FIG. 4(e) shows the
waveform of the signal IS2; and FIG. 4(f) shows the waveform of the
signal S3. As shown in FIG. 4(b) and FIG. 4(d), in the capacitive
region, the current of the series resonant circuit leads in phase
the voltage.
[0060] Since the capacitor 8 is coupled between the output end 3a
of the inverter circuit 3 and the detection point 4a, the phase of
the voltage at the detection point 4a (i.e., the signal IS2) shown
in FIGS. 3(e) and 4(e), respectively, leads by about 90.degree. the
phase of the current of the series resonant circuit 4 shown in
FIGS. 3(d) and 4(d), respectively. This is because when the voltage
at the detection point 4a is indicated by V.sub.IS2, the
relationship between the voltages Vout and V.sub.IS2 is expressed
by following Expression (5). In Expression (5), Zc denotes the
impedance of the capacitor 8, C denotes the capacitance of the
capacitor 8, and I denotes the current flowing through the series
resonant circuit 4.
V IS 2 = Vout - Zc I = Vout + j 1 .omega. C I [ Exp . 5 ]
##EQU00002##
In Expression (5), the voltage Vout denotes the output of the
inverter circuit 3, or either of the power source voltage VB or the
ground potential. When the voltage Vout is the ground potential,
the voltage V.sub.IS2 is
V IS 2 = j 1 .omega. C I [ Exp . 6 ] ##EQU00003##
and the phase of the voltage V.sub.IS2 leads by 90.degree. the
current I. When the voltage Vout is the power source voltage VB,
the voltage V.sub.IS2 is
V IS 2 = VB + j 1 .omega. C I [ Exp . 7 ] ##EQU00004##
and the phase of the voltage V.sub.IS2 leads the current I by an
angle expressed by following Expression (8)
.theta. = tan - 1 ( I VB .omega. C ) [ Exp . 8 ] ##EQU00005##
When the duty ratio of the drive signals Sdrv.sub.1, Sdrv.sub.2
which are supplied from the bridge driver 6 to the inverter circuit
3 is 50%, therefore, the phase of the voltage V.sub.IS2 leads the
current I over one period by
.THETA. = 90 .degree. + .theta. 2 [ Exp . 9 ] ##EQU00006##
[0061] In the case of the discharge lamp lighting circuit shown in
FIG. 1, during a period when the transfer to an arc discharge is
promoted immediately after lighting of the discharge lamp L, the
discharge lamp L has a high resistance, and hence the impedance of
the primary winding 7a of the transformer 7 is high, and the
current I is increased. Therefore, .theta. in Expression (8) is
approximately 90.degree., and .THETA. in Expression (9) is about
90.degree.. As described later, the frequency following controlling
portion 11 in this embodiment controls the bridge driver 6 so that
the series resonant circuit 4 operates in the vicinity of the
resonant frequency, and hence 1/.omega.C in Expressions (7) and (8)
is a value which is sufficiently larger than VB. Consequently,
.theta. in Expression (8) is approximately 90.degree., and .THETA.
in Expression (9) is about 90.degree.. For the above-described
reason, the phase of the voltage V.sub.IS2 at the detection point
4a shown in FIGS. 3(e) and 4(e) leads by about 90.degree. the phase
of the current I of the series resonant circuit 4 shown in FIGS.
3(d) and 4(d).
[0062] The reason that the current I is increased when the
resistance of the discharge lamp L is high (the impedance of the
primary winding 7a is high) will be described as follows. FIG. 5
shows a circuit diagram showing an equivalent circuit of the
resonance portion of the series resonant circuit 4. In FIG. 5, C
denotes the capacitance of the capacitor 8, Lr denotes the
inductance of the inductor 9, Lp denotes the inductance of the
primary winding 7a of the transformer 7, Ls denotes the inductance
of the secondary winding 7b, RL denotes the resistance of the
discharge lamp L, N denotes the turn ratio of the primary and
secondary windings 7a, 7b of the transformer 7, k denotes a
coupling constant of the transformer 7, IL denotes the lamp
current, and I denotes the exciting current flowing through the
primary winding 7a. In the equivalent circuit, the sum of I and IL
is the resonant current. The relationship is expressed by the
following Expression (10):
I = RL + j.omega. ( 1 - k ) Ls N 2 .omega. k Lp N IL [ Exp . 10 ]
##EQU00007##
Expression (10) above indicates that, when the resistance RL of the
discharge lamp L is high, the current I is increased.
[0063] As described above, the phase of the signal IS2 leads by
about 90.degree. the phase of the current I of the series resonant
circuit 4. Therefore, the first signal producing portion 14 further
advances the phase of the signal IS2 by 90.degree., then digitizes
the signal to produce a signal in which the phase difference with
respect to the current I of the series resonant circuit 4 is
180.degree., and inverts the resulting signal, thereby producing
the signal S3 (see FIGS. 3(f) and 4(f)) in which the phase
difference with respect to the current I is 0.degree..
[0064] The frequency following controlling portion 11 can determine
whether the operation state of the series resonant circuit 4 is in
the inductive region or in the capacitive region (i.e., whether the
current waveform of the series resonant circuit 4 lags or leads the
output waveform of the inverter circuit 3), in the following
manner. As shown in FIGS. 3(a) to 3(f), when the signal S3 is at
the low (L) level when the signal S4 rises to the high (H) level,
it is determined that the operation state of the series resonant
circuit 4 is in the inductive region. Furthermore, it is determined
that, the operation state more deeply enters the inductive region
as the zone T3 where the signal S3 is at the L level in the half
period T1 where the signal S4 is at the H level is longer. Also in
the case where the signal S3 is at the H level when the signal S4
falls to the L level, it is determined that the operation state of
the series resonant circuit 4 is in the inductive region.
Furthermore, it is determined that the operation state more deeply
enters the inductive region as the zone T4 where the signal S3 is
at the H level in the half period T2 where the signal S4 is at the
L level is longer.
[0065] As shown in FIGS. 4(a) to 4(f), when the signal S3 is at the
H level when the signal S4 rises to the H level, it is determined
that the operation state of the series resonant circuit 4 is in the
capacitive region. Furthermore, it is determined that, the
operation state more deeply enters the capacitive region as the
zone T5 where the signal S3 is at the L level in the half period T1
where the signal S4 is at the H level is longer. Also in the case
where the signal S3 is at the L level when the signal S4 falls to
the L level, it is determined that the operation state of the
series resonant circuit 4 is in the capacitive region. Furthermore,
it is determined that the operation state more deeply enters the
capacitive region as the zone T6 where the signal S3 is at the H
level in the half period T2 where the signal S4 is at the L level
is longer.
[0066] In the case where the frequency following controlling
portion 11 determines that the operation state of the series
resonant circuit 4 is in the inductive region, the frequency
following controlling portion increases the voltage level of the
control signal S1, and lowers the driving frequency of the bridge
driver 6, thereby causing the phase difference between the output
voltage Vout of the inverter circuit 3 and the current I of the
series resonant circuit 4 to approach zero. In the case where the
frequency following controlling portion 11 determines that the
operation state of the series resonant circuit 4 is in the
capacitive region, the frequency following controlling portion
decreases the voltage level of the control signal S1, and raises
the driving frequency of the bridge driver 6, thereby causing the
phase difference between the output voltage Vout of the inverter
circuit 3 and the current I of the series resonant circuit 4 to
approach zero. In this way, the frequency following controlling
portion 11 produces the control signal S1 so that the phase
difference between the output voltage Vout of the inverter circuit
3 and the current I of the series resonant circuit 4 approaches
zero, whereby the driving frequency of the bridge driver 6 is
caused to follow the series resonant frequency. The configuration
and operation of the frequency following controlling portion 11
will be described later in more detail.
[0067] Now, the operation of the discharge lamp lighting circuit 1,
shown in FIG. 1, will be described with reference to FIGS.
6(a)-6(d), FIGS. 7(a)-7(c) and FIG. 8. FIG. 6(a) shows the driving
frequency of the bridge driver 6, FIG. 6(b) shows the voltage of
the starting capacitor 51, and FIG. 6(c) shows the lamp voltage VL.
FIG. 7(a) shows the control signal Sc.sub.1 and FIG. 7(b) shows the
supplied power. Furthermore, FIG. 6(d) and FIG. 7(c) show
transitions of the control mode of the controlling portion 10. FIG.
8 is a graph showing relationships between the driving frequency of
the series resonant circuit 4 and the level of the supplied power
(or the OCV).
[0068] When the discharge lamp lighting circuit 1, shown in FIG. 1,
is first powered on (time t.sub.1), the driving frequency rises to
the maximum value as shown in FIG. 6(a). At this time, in the
controlling portion 10, the control signal S2 from the power
controlling portion 12 is selected and output as the control signal
Sc.sub.1. The driving frequency is controlled by the control signal
Sc.sub.1 to be converged to a value f.sub.1 at time t.sub.2 (OCV
control mode). The value f.sub.1 may be predetermined. The
relationships between the driving frequency of the series resonant
circuit 4 and the supplied power before lighting are indicated by
graph G1 shown in FIG. 8. The OCV according to the operating point
P1 corresponding to the driving frequency f.sub.1 is applied to the
discharge lamp L. The OCV may be predetermined. During this period,
the charging of the starting capacitor 51 of the starting portion 5
is started.
[0069] Thereafter, the voltage across the starting capacitor 51
reaches a threshold value (which may be predetermined), and the
self-breakdown switching element 52 is turned on (the time t.sub.3
in FIG. 6(b)). As shown in FIG. 6(c), then the starting portion 5
applies the high-voltage pulse P to the discharge lamp L. At this
time, a discharge between the electrodes of the discharge lamp L is
started to set the conductive state, and the lamp voltage VL is
lowered. In the controlling portion 10, the switch 131 is switched
so that the frequency following controlling portion 11 begins to
supply the control signal S1. The control signal S1 is output as
the control signal Sc.sub.1 from the controlling portion 10. When
the conductive state is set between the electrodes of the discharge
lamp L, the relationships shown in FIG. 8 between the driving
frequency of the series resonant circuit 4 and the supplied power
are transferred to the graph G2.
[0070] Namely, because of the conductive state due to the start of
a discharge in the discharge lamp L, the resonant frequency of the
series resonant circuit 4 becomes higher than the frequency
f.sub.1, and thereafter is continuously transferred from the lower
frequency to a higher frequency f.sub.2 as shown in FIG. 8. In
other words, the correlation graph G2 of the driving frequency and
the supplied power after lighting is continuously moved from the
low frequency side to the graph G3 on the high frequency side. The
frequency following controlling portion 11 supplies the control
signal S1 so that the driving frequency follows the change of the
resonant frequency. Therefore, the operating point is transferred
from P1 corresponding to the frequency f.sub.1 to P3 corresponding
to the frequency f.sub.2 while following the series resonant
frequency which is transferred to the high frequency side
(frequency following control mode).
[0071] After elapse of a time period of several milliseconds (the
time period may be predetermined) from application of the
high-voltage pulse to the discharge lamp L (time t.sub.4), the
switch 131 of the controlling portion 10 is again switched so that
the control signal S2 output from the power controlling portion 12
is again output as the control signal Sc.sub.1 (power control
mode). Thereafter, the power controlling portion 12 produces the
control signal Sc.sub.1 so that the level of the power to be
supplied to the discharge lamp L becomes close to the steady-state
value, and, as shown in FIG. 8, the operating point is stabilized
at the steady point P4 in the inductive region.
[0072] Hereinafter, a specific configuration example and operation
of the frequency following controlling portion 11, and first and
second signal producing portion 14, 15 according to an exemplary
embodiment of the present invention will be described.
[0073] FIG. 9(a) shows a block diagram showing an example of the
internal configurations of the frequency following controlling
portion 11, and the first and second signal producing portions 14,
15. As shown in FIG. 9(a), the frequency following controlling
portion 11 in this exemplary embodiment comprises a phase
difference detecting portion 111 and a signal converting portion
112. The first signal producing portion 14 comprises a
differentiating circuit 141 and a comparator 142. The second signal
producing portion 15 comprises a waveform-shaping circuit 151.
[0074] An input end 141a of the differentiating circuit 141 is
coupled to the input end 10c (see FIG. 2) of the controlling
portion 10 via the input end 14a of the first signal producing
portion 14, and the signal IS2 is supplied to the input end 141a.
An output end 141b of the differentiating circuit 141 is coupled to
one input end 142a of the comparator 142, and the differentiating
circuit 141 supplies a signal Sd1 which is obtained by
differentiating the signal IS2, to the comparator 142. The
differentiating circuit 141 is realized by a circuit configuration
such as shown in FIG. 9(b). The differentiating circuit 141 shown
in FIG. 9(b) has a capacitor 141c and a resistor element 141d. One
end of the capacitor 141c is coupled to the input end 141a, and the
other end is coupled to the output end 141b. The other end of the
capacitor 141c is coupled also to the ground potential via the
resistor 141d.
[0075] The comparator 142 is the converting circuit in this
exemplary embodiment, and converts the output signal Sd1 of the
differentiating circuit 141 to a digital signal. A threshold
voltage V3 is supplied to the other input end 142b of the
comparator 142. The threshold voltage may be predetermined. When
the signal Sd1 supplied to one input end 142a is larger than the
threshold voltage V3, the comparator 142 outputs the H level, and,
when the signal Sd1 is smaller than the threshold voltage V3, the
comparator 142 outputs the L level. An output end 142c of the
comparator 142 is coupled to the input end 11a of the frequency
following controlling portion 11 via an inverter (i.e., a NOT
circuit) 143 and the output end 14b of the first signal producing
portion 14, and an output of the NOT circuit 143 is provided as the
signal S3 to the frequency following controlling portion 11.
[0076] The waveform-shaping circuit 151 is a circuit which
waveform-shapes the signal VS2 to a rectangular waveform to
digitize the signal. An input end 151a of the waveform-shaping
circuit 151 is coupled to the input end 10d (see FIG. 2) of the
controlling portion 10 via the input end 15a of the second signal
producing portion 15, and the signal VS2 is supplied to the input
end 151a. An output end 151b of the waveform-shaping circuit 151 is
coupled to the input end 11b of the frequency following controlling
portion 11 via the output end 15b of the second signal producing
portion 15, and the waveform-shaping circuit 151 supplies the
signal S4 which is obtained by waveform-shaping the signal VS2, to
the frequency following controlling portion 11. For example, the
waveform-shaping circuit 151 may be realized by a clamp circuit and
a comparator.
[0077] As described above, the signals S3, S4 are supplied to input
ends 111a, 111b of the phase difference detecting portion 111,
respectively. When the phase of the signal S3 lags that of the
signal S4, the phase difference detecting portion 111 produces an
inductive detection signal S5 having a pulse width corresponding to
the phase difference (i.e., the width of the zone T3 or T4 shown in
FIGS. 3(a) to 3(f)). When the phase of the signal S3 leads that of
the signal S4, the phase difference detecting portion 111 produces
a capacitive detection signal S6 having a pulse width corresponding
to the phase difference (i.e., the width of the zone T5 or T6 shown
in FIGS. 4(a) to 4(f)). The inductive detection signal S5 indicates
that the operation state of the series resonant circuit 4 is in the
inductive region, and the capacitive detection signal S6 indicates
that the operation state of the series resonant circuit 4 is in the
capacitive region. The inductive detection signal S5 is supplied
from an output end 111c of the phase difference detecting portion
111 to an input end 112a of the signal converting portion 112, and
the capacitive detection signal S6 is supplied from an output end
111d of the phase difference detecting portion 111 to an input end
112b of the signal converting portion 112.
[0078] The signal converting portion 112 produces the control
signal S1 on the basis of the inductive detection signal S5 and the
capacitive detection signal S6. When the inductive detection signal
S5 having a certain pulse width is input, the signal converting
portion 112 raises the voltage level of the control signal S1, and,
when the capacitive detection signal S6 having a certain pulse
width is input, lowers the voltage level of the control signal S1.
An output end 112c of the signal converting portion 112 is coupled
to the selecting portion 13 (see FIG. 2) via the output end 11c of
the frequency following controlling portion 11, and the control
signal S1 is supplied to the selecting portion 13.
[0079] The effects of the above-described discharge lamp lighting
circuit according to an exemplary embodiment of the present
invention will now be described. In the discharge lamp lighting
circuit 1, the controlling portion 10 which controls the driving
frequency of the inverter circuit 3 comprises the first signal
producing portion 14 for detecting the phase of the current I
flowing through the series resonant circuit 4; and the second
signal producing portion 15 for detecting the phase of the output
voltage (AC voltage) Vout supplied from the inverter circuit 3. The
controlling portion 10 controls the driving frequency on the basis
of the phase difference between the current I flowing through the
series resonant circuit 4 and the voltage Vout. According to this
exemplary configuration, it is possible to operate the
above-described frequency following control mode (i.e., a mode in
which the driving frequency is controlled so that the phase
difference between the voltage Vout and the current I approaches
zero, and the driving frequency of the inverter circuit 3 is made
coincident with the resonant frequency of the series resonant
circuit 4, so that the maximum power of the series resonant circuit
4 is supplied to the discharge lamp L).
[0080] In the discharge lamp lighting circuit 1 of this exemplary
embodiment, among the inductor 9, the transformer 7, and the
capacitor 8, only the capacitor 8 is coupled between the detection
point 4a coupled to the first signal producing portion 14 and the
output end 3a of the inverter circuit 3. FIG. 10 is a view
diagrammatically showing a configuration of a discharge lamp
lighting circuit according to an exemplary embodiment of the
present invention. In FIG. 10, a DC power source 41, a bridge
driver 42, and an inverter circuit 40 (transistors 43, 44) are
configured in the same manner as the DC power source B, the bridge
driver 6, and the inverter circuit 3 (transistors 31, 32 in
above-described exemplary embodiment). The blocks 45, 46, 47 are an
inductor, a transformer, and a capacitor, respectively, and are
coupled in series in this sequence between the output ends 40a, 40b
of the inverter circuit 40. It is assumed that the impedances of
the blocks 45, 46, 47 are Z1, Z2, and Z3, respectively.
[0081] When the voltage of the output end 40a of the inverter
circuit is indicated by Va, and the voltage at a position after the
block 45 is coupled is indicated by Vb, the relationship between
the voltages Va and Vb is expressed by following Expression (11).
In Expression (11), I denotes the current flowing through the
blocks 45, 46, 47.
[0082] [Exp. 11]
Vb=Va-Z1I (11)
[0083] In Expression (11), the potential Va denotes the output of
the inverter circuit 40, and any one of the power source voltage
and the grounding potential. According to Expression (11),
therefore, the value of the current I can be obtained from that of
the voltage Vb. In other words, the phase of the current I can be
known by detecting the voltage Vb in the series resonant
circuit.
[0084] In the discharge lamp lighting circuit this exemplary
embodiment, the capacitor 8 is placed at the position of the block
45. As shown in FIGS. 3(f) and 4(f), therefore, the phase of the
current I can be obtained by referring to the voltage Vb, i.e., the
voltage signal IS2 at the detection point 4a in the series resonant
circuit 4. Accordingly, a transformer or resistor for detecting a
current is not required in the series resonant circuit 4, and the
phase of the current I of the series resonant circuit 4 can be
accurately detected even in a state where the discharge lamp L is
not lighted, or an arc discharge has not yet occurred.
[0085] Alternatively, the circuit element which is placed at the
position of the block 45 may be an element other than the
capacitor, for example the inductor or the primary winding of the
transformer. When one of these circuit elements is placed at the
position of the block 45, the phase of the current I of the series
resonant circuit 4 can also be detected.
[0086] The detection point to which the first signal producing
portion 14 is coupled is not restricted to the detection point 4a
in the above-described exemplary embodiment, and the detection
point may alternatively be located between the inductor 9 and the
primary winding 7a of the transformer 7. In FIG. 10, the voltage Vc
of the output end 40b of the inverter circuit 40 is 0 (the
grounding potential). When the voltage of the position at an output
end of the block 47 is indicated by Vd, therefore, the voltage Vd
is expressed by following Expression (12).
[0087] [Exp. 12]
Vd=Z3I (12)
According to Expression (12), the value of the current I can be
obtained from that of the voltage Vd. In other words, the phase of
the current I can be known by detecting the voltage Vd in the
series resonant circuit. In this exemplary embodiment, the primary
winding 7a of the transformer 7 is placed at the position of the
block 47. Therefore, the phase of the current I of the series
resonant circuit 4 can also be detected by referring to the voltage
Vd, i.e., the voltage between the inductor 9 and the primary
winding 7a of the transformer 7. In this case, the circuit element
which is placed at the position of the block 47 may be an element
other than the transformer, for example the capacitor or the
inductor.
[0088] In the above-described exemplary embodiment, the element
which is placed between the output end 3a (or 3b) of the inverter
circuit 3 and the detection point (4a in this exemplary embodiment)
is a capacitor. Usually, an inverter circuit is configured by a
transistor, and an element which is of the surface mount type and
which has a small size is often used as the transistor. Similarly,
a capacitor which is of the surface mount type and which is
relatively smaller than an inductor and a transformer can be used.
When a capacitor is disposed in place of an inductor or a
transformer between the detection point 4a to which the first
signal producing portion 14 is coupled and the output end 3a of the
inverter circuit 3, therefore, the current path of the series
resonant circuit 4 can be shortened, and the high-frequency
characteristic of the series resonant circuit 4 can be stabilized.
Furthermore, the transistors 31, 32 and capacitor 8 which are small
in size can be disposed close to each other, so that the space on a
circuit board can be efficiently used.
[0089] As shown in FIG. 9(a) and 9(b), the first signal producing
portion 14 comprises the differentiating circuit 141 which
differentiates the voltage signal IS2 at the detection point 4a;
and the converting circuit (the comparator 142) which converts the
output signal Sd1 of the differentiating circuit 141 to a digital
signal. In the discharge lamp lighting circuit 1 of this exemplary
embodiment, the capacitor 8 is coupled between the output end 3a of
the inverter circuit 3 and the detection point 4a, and hence the
phase of the voltage signal IS2 at the detection point 4a leads by
about 90.degree. the phase of the current I. Also in the case where
the driving frequency is controlled so that the phase difference
between the voltage Vout and the current I approaches zero, and the
driving frequency of the inverter circuit 3 is made coincident with
the resonant frequency of the series resonant circuit 4, the phase
of the voltage signal IS2 leads by about 90.degree. the current I.
In these cases, when the voltage signal IS2 at the detection point
4a is differentiated, the phase difference between the
differentiated voltage signal IS2 and the current I is about
180.degree., or the waveform of the differentiated voltage signal
IS2 is an inversion of that of the current I. Therefore, the phase
difference can be made about 0.degree. by a simple circuit such as
the NOT circuit 143 (see FIGS. 3(f) and 4(f)), and hence the phase
difference between the voltage signal IS2 after converted to a
digital signal, and the current I can be easily processed.
(First Modification)
[0090] FIGS. 11(a) and 11(b) show a block diagram of another
exemplary embodiment of the present invention. FIG. 11(a) shows an
example of the internal configurations of the frequency following
controlling portion 11, a first signal producing portion 16, and
the second signal producing portion 15. The configurations of the
frequency following controlling portion 11 and the second signal
producing portion 15 are the same as those of the exemplary
embodiment described above, and hence their detailed description is
omitted.
[0091] The first signal producing portion 16 comprises an
integrating circuit 161 and an comparator 162. The signal IS2 is
supplied to an input end 161a of the integrating circuit 161 via an
input end 16a of the first signal producing portion 16. An output
end 161b of the integrating circuit 161 is coupled to one input end
162a of the comparator 162, and the integrating circuit 161
supplies a signal Si1 which is obtained by integrating the signal
IS2, to the comparator 162. For example, the integrating circuit
161 is realized by a circuit configuration such as shown in FIG.
11(b). The integrating circuit 161 shown in FIG. 11(b) has a
resistor element 161c and a capacitor 161d. One end of the resistor
element 161c is coupled to the input end 161a, and the other end is
coupled to the output end 161b. The other end of the resistor
element 161c is coupled also to the ground potential via the
capacitor 161d.
[0092] The comparator 162 is the converting circuit, and converts
the output signal Si1 of the integrating circuit 161 to a digital
signal. The threshold voltage V3 (which may be predetermined) is
supplied to the other input end 162b of the comparator 162. When
the signal Si1 supplied to the one input end 162a is larger than
the threshold voltage V3, the comparator 162 outputs the signal S3
of the H level, and, when the signal Si1 is smaller than the
threshold voltage V3, the comparator 162 outputs the signal S3 of
the L level. An output end 162c of the comparator 162 is coupled to
the input end 11a of the frequency following controlling portion 11
via the output end 16b of the first signal producing portion 16,
and the signal S3 is provided to the frequency following
controlling portion 11.
[0093] The first signal producing portion 16 comprises the
integrating circuit 161 which integrates the voltage signal IS2 at
the detection point 4a; and the converting circuit (the comparator
162) which converts the output signal Si1 of the integrating
circuit 161 to the digital signal S3. As described above, the phase
of the voltage signal IS2 at the detection point 4a leads by about
90.degree. the current I. When the voltage signal IS2 at the
detection point 4a is integrated as in the modification, the phase
difference between the integrated voltage signal IS2 and the
current I is about 0.degree.. Therefore, the phase difference
between the voltage signal IS2 after converted to a digital signal,
and the current I can be easily processed.
(Second Modification)
[0094] FIGS. 12(a) and 12(b) are block diagrams of yet another
exemplary embodiment of the present invention. FIG. 12(a) shows an
example of the internal configurations of the frequency following
controlling portion 11, a first signal producing portion 17, and
the second signal producing portion 15. The configurations of the
frequency following controlling portion 11 and the second signal
producing portion 15 are the same as those of the exemplary
embodiment described above and will therefore be omitted.
[0095] The first signal producing portion 17 comprises an
integrating circuit 171, a differentiating circuit 172, and an
comparator 173. The integrating circuit 171 is the first circuit
which performs integration on the signal IS2. The signal IS2 is
supplied to an input end 171a of the integrating circuit 171 via an
input end 17a of the first signal producing portion 17. An output
end 171b of the integrating circuit 171 is coupled to an input end
172a of the differentiating circuit 172. The integrating circuit
171 supplies a signal Si2 which is obtained by integrating the
signal IS2, to the differentiating circuit 172. The differentiating
circuit 172 is the second circuit which performs differentiation on
the output signal Si2 of the integrating circuit 171. An output end
172b of the differentiating circuit 172 is coupled to one input end
173a of the comparator 173, and the differentiating circuit 172
supplies a signal Sd2 which is obtained by differentiating the
signal Si2, to the comparator 173. For example, the integrating
circuit 171 and the differentiating circuit 172 may be realized by
a circuit configuration such as shown in FIG. 12(b). The
integrating circuit 171 shown in FIG. 12(b) has a resistor element
171c and a capacitor 171d. The connection relationship of the
circuit is the same as the integrating circuit 161 in FIG. 11(b)
described above. The differentiating circuit 172 has a capacitor
172c and a resistance element 172d. The connection relationship of
the circuit is the same as that of the differentiating circuit 141
shown in FIG. 9(b) described above. The output end 172b of the
differentiating circuit 172 is clamped by a diode 174.
[0096] The comparator 173 is the converting circuit, and converts
the output signal Sd2 of the differentiating circuit 172 to a
digital signal. The threshold voltage V3 (which may be
predetermined) is supplied to another input end 173b of the
comparator 173. When the signal Sd2 supplied to the one input end
173a is larger than the threshold voltage V3, the comparator 173
outputs the signal S3 of the H level, and, when the signal Sd2 is
smaller than the threshold voltage V3, the comparator 173 outputs
the signal S3 of the L level. An output end 173c of the comparator
173 is coupled to the input end 11a of the frequency following
controlling portion 11 via the output end 17b of the first signal
producing portion 17, and the signal S3 is provided to the
frequency following controlling portion 11.
[0097] In the case where the signal IS2 is differentiated by the
differentiating circuit 141, the differentiating circuit cuts the
DC component of the input signal, and hence detection of a zero
cross of the signal IS2 is highly accurate. When the signal IS2
contains high-frequency noise, however, the differentiating circuit
tends to allow the noise components to pass therethrough because
the gain is higher as the frequency is higher, and hence erroneous
detection may be caused. By contrast, in the case where the signal
IS2 is integrated by the integrating circuit 161 as in the first
modification, when the input signal contains high-frequency noise,
the integrating circuit cuts the noise components, and the S/N
ratio of the signal IS2 can be improved. In order to realize a
phase lag, however, the gain is excessively lowered, and there is a
possibility that a signal cannot be detected. When the integrating
circuit 171 and the differentiating circuit 172 are combined with
each other and the circuits are set to respective adequate cutoff
frequencies, the S/N ratio can be improved while the accuracy of
detection of a zero crossing of the signal IS2 is enhanced. In
another exemplary embodiment of the present invention, the
differentiating circuit may be placed in the preceding stage of the
integrating circuit. In this case, the differentiating circuit is
the first circuit which performs differentiation on the signal IS2,
and the integrating circuit is the second circuit which performs
integration on an output signal of the differentiating circuit.
(Third Modification)
[0098] FIG. 13 is a block diagram showing a discharge lamp lighting
circuit according to yet another exemplary embodiment of the
present invention. FIG. 13 shows a discharge lamp lighting circuit
1a which is different from above-described exemplary embodiments in
that the placement of the capacitor and the inductor is different.
In the discharge lamp lighting circuit 1a, the inductor 9, the
capacitor 8, and the primary winding 7a of the transformer 7 of a
series resonant circuit 48 are coupled in series in this sequence.
One end of the series circuit on the side of the inductor 9 is
coupled to the one output end 3a of the inverter circuit 3, and the
other end on the side of the primary winding 7a is coupled to the
other output end 3b of the inverter circuit 3.
[0099] A detection point 48a for referring to the signal IS2 is set
between the inductor 9 and the capacitor 8. In other words, among
the inductor 9, the transformer 7, and the capacitor 8, the
inductor 9 is coupled between the detection point 48a and the
output end 3a of the inverter circuit 3.
[0100] The inductor 9 is placed at the position of the block 45 in
the diagram of FIG. 10, and the phase of the current I is detected
by referring to the voltage Vb. The circuit element which is placed
at the position of the block 45 may be the inductor. Although not
illustrated, alternatively, the circuit element may be the primary
winding of the transformer. When one of these circuit elements is
placed at the position of the block 45, the phase of the current I
of the series resonant circuit 48 can be detected.
(Fourth Modification)
[0101] FIG. 14 is a block diagram showing a configuration of a
discharge lamp lighting circuit according to yet another exemplary
embodiment of the present invention. A discharge lamp lighting
circuit 1b is different from the above-described exemplary
embodiments in the placement of the capacitor and the position of
the detection point. In the discharge lamp lighting circuit 1b, the
inductor 9, the primary winding 7a of the transformer 7, and the
capacitor 8 of a series resonant circuit 49 are coupled in series
in this sequence. One end of the series circuit on the side of the
inductor 9 is coupled to the one output end 3a of the inverter
circuit 3, and the one end on the side of the capacitor 8 is
coupled to the other output end 3b of the inverter circuit 3. A
detection point 49a for referring to the signal IS2 is set between
the primary winding 7a and the capacitor 8. In other words, among
the inductor 9, the transformer 7, and the capacitor 8, the
capacitor 8 is coupled between the detection point 49a and the
output end 3b of the inverter circuit 3.
[0102] The capacitor 8 is placed at the position of the block 47 in
the diagram of FIG. 10, and the phase of the current I is detected
by referring to the voltage Vd. The circuit element which is placed
at the position of the block 47 may be the inductor or the primary
winding of the transformer. When one of these circuit elements is
placed at the position of the block 47 and the voltage Vd (i.e.,
the voltage at the detection point 49a) is referred to, the phase
of the current I of the series resonant circuit 49 can be
detected.
* * * * *